1. Field of the Invention
The present invention relates to a closed-loop control device and, more particularly, to a control device having voltage and current detection modules mounted between a DC motor and a processor and receiving operating voltage of the DC motor as feedback to the processor for the processor to control the operating voltage of the DC motor through closed-loop control, and to a method for controlling the control device.
2. Description of the Related Art
Sewing machines in current market include computerized sewing machines and mechanical sewing machines. Computerized sewing machines employ software programs to control a rotation encoder for sewing operation. Despite the availability of embroidery function, computerized sewing machines are usually more costly. As there is no embroidery function, mechanical sewing machines are less costly relative to computerized sewing machines. Generally, the sewing speed of mechanical sewing machines is adjusted by users to control the pedal of the sewing machine for adjustment of a rotation angle or height of the pedal. Thus, the rotation speed of the DC (Direct Current) motor of mechanical sewing machines or the sewing speed can be changed. However, as the sewing speed of mechanical sewing machines is adjusted depending on users' habit or sewing demand, mechanical sewing machines usually have no precise control over the sewing speed.
With reference to FIG. 7, an equivalent circuit of a DC motor is shown with Vm representing an operating voltage, Ra representing an internal resistance, Ia representing an armature current, and E representing an induced EMF (Electromotive Force) generated by an armature when cutting a magnetic field, and an equation associated with the foregoing parameters can be expressed as follows:Vm=IaRa+E 
The induced EMF E can be further expressed as:E=KΦN 
where K is a winding factor of the armature, Φ is magnetic flux, and N is a rotation speed of the DC motor.
From the foregoing two equations, the rotation speed of the DC motor can be expressed as:
  N  =            Vm      -      IaRa              K      ⁢                          ⁢      Φ      
As can be seen from the above equation, the rotation speed N of the DC motor can be adjusted by controlling the operating voltage Vm.
The rotation speed of the DC motor in conventional mechanical sewing machines can be adjusted by varying the rotation angle or height of the pedal to change the operating voltage Vm of the DC motor. However, when users intend to apply fixed operating voltage Vm for the DC motor to rotate at a constant rotation speed, a counter EMF will prevent the armature winding from rotating. To keep the armature winding running at a constant speed, additional power needs to be provided to allow the DC motor to run at a constant speed. Such need of additional power causes the armature current Ia to rise. When the rotation speed of the DC motor is constant, the rising armature current Ia will cause the operating voltage Vm to drop. Such voltage drop of the operating voltage Vm further leads to a lower rotation speed of the DC motor. In turn, to maintain the constant rotation speed of the DC motor, the lower rotation speed of the DC motor should be compensated by tilting the pedal to increase the rotation angle of the pedal and then again increase the operating voltage Vm.
Therefore, the DC motor of the conventional mechanical sewing machines requires the use of high-voltage (HV) DC motor (HV DC motor) with high power for normal operation. This is why high armature current Ia and high operating voltage Vm of the HV DC motor become inevitable in the conventional mechanical sewing machines, and the outcome is the rise of operating temperature of the HV DC motor, which results in issues of short life duration, high power consumption, high operating temperature, and brush sparking of the HV DC motor attributable to a worse operational safety.